For a better understanding of problems and technical solutions currently known in connection with the locking of a stud on a hub, a conventional locking arrangement is briefly described, reference being made to FIG. 1 in the appended drawings.
Referring to FIG. 1, in order to fix the rim and the brake rotor to a radial flange 17 of the hub 18 of a wheel, most of the known solutions provide that the wheel (not shown), a flange of the brake rotor (not shown) and the radial flange 17 of the hub are axially flanked so as to align bores formed in these members. Usually, four or five bolts with studs 10 are employed, inserting the studs from the axially inner side (or inboard side) 22 of the hub flange. Each stud has a head 11 and a stem 12 with a threaded end portion 13 and a length 14 having an axial knurling near the head. The studs 10 are forcefully driven with radial interference into circular axial bores 16 formed in the flange 17 of the hub. After this forced insertion, first the brake member and then the wheel (not shown) are inserted from the outside on the end portions of the stud stems. Finally, outer nuts (not shown) are screwed and tightened with a predetermined torque. The knurlings 14 serve to rotationally lock the studs relative to the hub flange both when the aforesaid tightening torque is applied and when the nut is unscrewed for removing the wheel and/or the brake rotor.
Such locking effect can nevertheless fail due to the same forced driving step itself. In order that the knurling may engrave the material of the flange, the studs are previously hardened and tempered. However, the crests of a knurling are particularly difficult to harden in that, being sharp parts, they tend to decarburize. Therefore, in being forcefully driven as said, the crests of the knurling are abraded and the anti-rotation coupling loses its efficiency. This problem, besides being uncontrollable, appears when in attempting to tighten the nuts, the studs rotate, rendering this operation difficult. In addition, for the anti-rotation coupling to be efficient, the steel of which the hub is made must be considerably less hard than the material of which the studs are made.
A further problem concerning the driving lies in that, owing to the considerable radial interference required to prevent the studs from rotating relative to the hub, lumps of material 20 removed from the bores 16 are inevitably formed on the axially outer face 19 of the hub flange. Volcano-shaped formations of this kind are undesired as they constitute protrusions on the outer face 19. Instead, this face should ideally provide a completely flat surface against which the brake member abuts. Furthermore, the high axial driving forces cause further deformation in form of radial undulations on the axially outer face of the hub flange.
Owing to all of these factors, the outer face 19 does no longer provide a flat resting surface perpendicular to the axis of rotation. As a result, anomalous vibration (the so-called “juddering”) occurs in operation.
As motor vehicle manufactures require to reduce to a minimum the so-called axial runout of the surface of the hub that serves as an axial rest for the brake rotor, after driving the studs it is necessary to subject the resting surface 19 to a finishing step on the lathe so as to eliminate or at least reduce the undesired effects of the forced driving. This finishing step is expensive, involves a long working time and is also made difficult owing to the presence of the studs driven through the hub flange. In order to obviate at least some of these problems, it has been proposed to form a recessed annulus embracing the zones where the bores 16 open onto the resting surface 19, so as to reduce or eliminate the adverse effect of the lumps 20. However, the undulations remain, and therefore a finishing step on the lathe is practically essential if axial runout is to be reduced to values being less than 30 μm.